The present invention relates to a method for decreasing the content of alkoxy groups in an alkoxy-containing organopolysiloxane or, more particularly, to a method for decreasing the content of residual alkoxy groups in an organopolysiloxane having alkoxy groups bonded to the silicon atoms so as to give an organopolysiloxane with an extremely low content of residual alkoxy groups even when the organopolysiloxane comprises tetrafunctional siloxane units of the unit formula SiO.sub.2 to be useful as a reinforcing agent of silicone rubbers.
Organopolysiloxanes called an MQ resin soluble in organic solvents consisting of monofunctional siloxane units of the formula R.sub.3 SiO.sub.0.5, referred to as M units hereinbelow, in which R is a monovalent hydrocarbon group, and tetrafunctional siloxane units of the formula SiO.sub.2, referred to as Q units hereinbelow, are well known in the art of silicones and are practically used in a variety of silicone products. Such an MQ resin is prepared by hydrolyzing a water-soluble basic silicate such as sodium orthosilicate or so-called water glass with addition of an inorganic acid such as hydrochloric and sulfuric acids to be converted into a silicic acid oligomer which is then reacted with a trialkyl chlorosilane (see, for example, U.S. Pat. Nos. 2,676,182 and 2,814,601). This method, however, has several disadvantages and problems due to the relatively low stability of the silicic acid oligomer as the intermediate that difficulties are encountered in reproducibly controlling the molecular weight distribution and the molar ratio of the M units and Q units not to give a product as desired. The stability of the silicic acid oligomer can be improved by admixing the reaction mixture with a large amount of an alcohol but the organopolysiloxane obtained by this method unavoidably contains a substantial amount of alkoxy groups bonded to the silicon atoms so that the MQ resin is not always satisfactory for several applications.
Alternatively, organopolysiloxanes of this kind can be prepared by a method disclosed in U.S. Pat. No. 2,857,356, in which an alkyl silicate and a trialkyl chlorosilane are co-hydrolyzed in the presence of hydrochloric acid or by a method disclosed in Japanese Patent Kokai 61-195129, in which a hexaalkyl disiloxane or a trialkyl chlorosilane is reacted with an alkyl silicate or a partial hydrolysis product thereof in the presence of an aqueous hydrochloric acid. These methods are advantageous as compared with the above described method in respect of the relatively good reproducibility of the molar ratio of the M units to the Q units and the molecular weight distribution in the organopolysiloxane product. A problem in these methods again consists in the large amount of the residual alkoxy groups bonded to the silicon atoms because a large amount of an alcohol is produced from the starting materials in the course of the reaction or addition of an alcohol to the reaction mixture is necessary in order to ensure smooth proceeding of the reaction.
A further alternative method has been recently disclosed in Japanese Patent Kokai 63-256628, according to which an organosilane compound or an organopolysiloxane is reacted with an alkyl silicate or a partial hydrolysis product thereof in the presence of a sulfonic acid group-containing compound and/or phosphonitrile chloride as a catalyst. This method is also disadvantageous due to the residual alkoxy groups in the organopolysiloxane product although good reproducibility can be obtained in the control of the M:Q molar ratio and molecular weight distribution.
As is described above, none of the prior art methods is quite satisfactory in respect of the residual content of the alkoxy groups in the organopolysiloxane product greatly affecting the inherently excellent weatherability and thermal stability of silicone products in general. In addition, the residual alkoxy groups are subject to the condensation reaction taking place in the lapse of time decreasing the stability of the organopolysiloxane. As a consequence, the MQ resins prepared by the prior art method cannot be very effective as a reinforcing agent of silicone rubbers.
As a means to decrease the content of alkoxy groups in the organopolysiloxane, it would be a possible way that the reaction mixture for the preparation of the organopolysiloxane is admixed with an excess amount of water so as to accelerate the hydrolysis reaction of the alkoxy groups. This method, however, is not practical because the decrease in the content of the alkoxy groups by this method cannot be high enough even after a prolonged reaction time.